Hand held devices for the detection of analytes in a sample of, for example, urine, are known. The use of such devices as home tests for pregnancy and fertility is now commonplace and a wide variety of test devices and kits are available commercially. Many of these devices rely on capillary action. Thus, the use of strip material (e.g. a membrane) along which a liquid sample will travel by capillary action in devices for the detection of analytes by specific binding assays, such as immunoassays, has previously been proposed. Generally, a liquid sample is applied to the strip material and permeates throughout the strip material to a region impregnated with a specific binding partner for the analyte under test. The analyte bound to the specific binding partner migrates further along the strip where it is immobilized at an indicator region impregnated with an immobilizing agent specific for the analyte bound specific binding partner. The extent to which the analyte present in the sample becomes immobilized at the indicator region is determined by labelled reagents either incorporated in the strip or applied subsequently thereto. Generally, the presence of immobilized analyte at the indicator region provides a color change at the indicator region and it is the detection of that color change by the user that indicates the presence of the particular analyte in the sample. Such devices are widely used as home pregnancy and fertility tests.
A problem associated with known analyte assay devices is that there is scope for error associated with the reading of the color change. Generally the specific binding partner for the analyte is labelled with a visible colored label, for example latex particles impregnated with a dye. The indicator region of the test strip in a known device generally would contain an amount of an immobilizing agent which stops the analyte within the indicator zone so that the amount of label at the indicator zone builds up to give a visible color change. The presence of this immobilizing agent may provide a slight amount of color to the indicator region and it is this color that is intensified when a positive result is observed. For example, the strip at the indicator region may be pale blue. When the test has been used and the analyte has bound a labelled antibody, which is immobilized at the indictor region, the strip at the indicator region may be a darker blue. The color change is not easily reproducible or accurately readable by eye, especially under varying light conditions.
Furthermore, in tests which require reading a color change by eye, a user of an analyte assay device may have a preferred outcome to the assay in mind when reading the assay results and this may cloud their interpretation of the color change. For example, a user of a pregnancy test kit (such as based on an antigen/antibody binding reaction for the detection of hCG) which indicates pregnancy by an intensification of the color at the indicator region may see an intensification in color more readily if they wanted to be pregnant than if they did not want to be pregnant. This leads to error in determining the presence of the analyte and the condition which is associated with the presence of that analyte, in this case pregnancy.
Another factor can arise from the test continuing to run after the initial result is given. The test is optimized to give a result in a short time frame, 1-3 minutes. However the antibody-antigen binding reaction continues to occur as long as the test strip is wet and analyte can flow. Since every sample of female urine will contain a basal level of hCG, it is possible that over time sufficient color can build up to be detectable.
In order to overcome the above disadvantages, electrochemical detection has been proposed for pregnancy testing devices. Thus, for example, PCT publication WO-A-00 00827 discloses a device in which a specific binding partner for the analyte has a label which is directly or indirectly electrochemically detectable, the device further comprising an electrochemical detection arrangement. The electrochemically detectable label may, for example, be a P1 nuclease label, in which case the carrier of the device also incorporates all of the substrates and enzymes, other than P1 nuclease, necessary to generate hydrogen peroxide in a reaction catalyzed by P1 nuclease. The electrochemical detection arrangement serves to detect the presence (or otherwise) of hydrogen peroxide. The device of WO-A-00 00827 may incorporate a LED or LCD display device for illustrating the result of the test.
A further proposal using an electrochemical detection arrangement is disclosed in WO-A-0033063. The device of this WO specification comprises a substrate along which a liquid may travel by capillary action. Provided on the substrate is at least one pair of electrodes of dissimilar metal arranged such that liquid flowing along the substrate comes between the electrodes and causes a current to be generated for operating an electrochemical detection arrangement of the device. Once again, the result of the test may be displayed, for example, on a LCD display device.
A further assay device incorporating electronic, visually readable output means (e.g. an LCD) is disclosed in WO-A-9506240 (Metrika Laboratories, Inc.). This WO specification discloses a disposable self-contained, electronic assay device for use in determining the amount of one or more analytes in a body fluid such as blood or urine. The device comprises a card-like housing having a receiving region to which liquid sample is applied and a sample, treatment region to which the sample passes. The sample treatment region is capable of producing a physical detectable change if the analyte is present in the sample (or present above a predetermined amount). The change may, for example, be a color change. The device further comprises a detector, signal processing means and an electronic display device together with a power source for operating these three components. Any change at the sample treatment area (caused by the analyte) is sensed by the detector so that the signal processing means is able to determine the result of the test and provide a readout on the display device.
By way of further background to the present invention, reference is also made to WO-A-99 35497 (Bio-Diagnostics Limited) which discloses a device for testing liquids. The device specifically disclosed in WO-A-99 35497 is for identifying blood groups and comprises a co-operating plate and lid arrangement which together define a number of capillary channels, each having an upstream end into which blood to be tested is introduced and a downstream vent (to allow blood flow along the capillaries). The device incorporates three capillary channels which, part way along their lengths, are formed into one or other of the ‘indicator letters’ A, B or 0 (i.e. blood group designations). Upstream of the indicator letters, each capillary channel has an agglutination reagent system that will cause agglutination of blood which is of a type represented by the indicator letter of that channel. Thus, for example, the channel formed with ‘A’ as the ‘indicator letter’ incorporates an agglutination reagent system that will cause agglutination of blood type A (but not types B or 0). There is a further capillary track (along which blood may travel) which does not include an agglutination system and which has a ‘check mark’ instead of an ‘indicator letter’. Apart from the ‘indicator letters’ and the ‘check mark’ (all of which are initially colorless) the device otherwise has a red background.
To determine blood group type, a blood sample is introduced at the upstream ends of the capillary tracks. Blood should flow unhindered along the track associated with the ‘check mark’ which will then become colored red as an indication that the device is functioning properly. This track can therefore be considered as a ‘control track’.
Depending on the blood type of the sample under test, the blood will become agglutinated in one of the other three tracks but will flow along the other two. Thus, for example, if the blood group is of the type A then the blood will become agglutinated in the track having the ‘A’ as the ‘indicator letter’. Blood in this track is therefore not able to reach the ‘indicator letter’. In contrast, blood is able to flow fully along the other two tracks and fill the ‘indicator letters’ associated therewith. Thus after the test has been conducted two of the ‘indicator letters’ as well as the ‘check mark’ will have become colored red and will no longer be visible against the red background of the device. The remaining ‘indicator letter’ in the pathway in which blood has been agglutinated will however still remain clear and thus the blood type is determined.